In fairness at LHR I have never been judged (not so I'd know anyway) based on what I am wearing whist strolling into First to check in, and I do not posses a suit. Jeans, T-shirt and adidas every time. GCH, in whatever class.
On the other hand, during boarding waiting in the group 1 line, I have been shoulder charged out the way by (usually) a suit wearing 'gentleman' on numerous occasions. Except last week, where my partner spotted the shoulder charger coming and body blocked him before announcing 'this is a group 1 line, and you aren't in it' loudly enough for it not to go unheard by anyone. Takes no nonsense, does she!
Quite why BA don't have a staffer walk the various lines before boarding commences, checking boarding passes and politely redirecting those requiring such beats me. CX manage it well enough in HKG doing it just this way. Never any drama.